1. Field of the Invention
The present invention relates to the field of data compression.
2. Background Art
Compression is a scheme for reducing the amount of information required to represent data. Data compression schemes are used, for example, to reduce the size of a data file so that it can be stored in a smaller memory space. Data compression may also be used to compress data prior to its transmission from one site to another, reducing the amount of time required to transmit the data. To access the compressed data, it is first decompressed into its original form. A compressor/decompressor (codec) is typically used to perform the compression and decompression of data. One measure of the performance or efficiency of a codec is its "compression ratio". Compression ratio refers to the ratio of number of bits of uncompressed data to the number of bits of compressed data. Compression ratios may be 2:1, 3:1, 4:1 etc.
Data compression may also be required when the input/output rate of a particular data receiver is less than the data rate of the transmitted data. This can occur when providing video data to computer systems. Video data of frame size 320.times.240 is provided at rates approaching 7 megabytes per second. This rate is greater than the rate of commonly used I/O subsystems of personal computers. Some representative rates of common I/O subsystems found on personal computers (PC) are:
______________________________________ Serial Communications 1-2 kilobytes/sec; ISDN 8-16 kilobytes/sec; Ethernet/CD-ROM 150-300 kilobytes/sec; SCSI Disk 0.5-2 megabytes/sec. ______________________________________
Another measure of video codec compression ratio is the average compressed bits-per-pixel. This measure is useful in describing video compression because different conventions are used for calculating the size of uncompressed video, i.e., some use 24 bits-per-pixel RGB and others use 4:2:2 subsampled YUV (16-bits per pixel). The averaging accounts for potentially different strategies employed for frames in a sequence. The bandwidth requirements for a sequence of frames is calculated by multiplying the average compressed bits-per-pixel and the number of frames per second, and dividing the resulting product by the number of pixels in each encoded frame.
Nearly all video compression techniques are lossy, i.e., information is inevitably discarded in the compression process. A measure of quality is how much this information is noticed by a human observer. However, there is not a consistent, objective model of human perception that can be applied. A simple, concrete, quality metric that is frequently used is the Mean-Squared-Error (MSE) that measures the error on a per-pixel basis from the uncompressed original.
Most compression algorithms are computationally complex, which limit their application since very complex algorithms often require expensive hardware to assist in the compression. A useful number to measure computational complexity of software-based compression algorithms is MIPS per megapixels/sec, i.e., essentially instructions/pixel. For example, an algorithm just capable of compressing 320.times.240 pixels per frame at 30 frames per second on a 40 MIPS machine has a computational complexity of 40,000,000/(320.times.240.times.30).congruent.17 instructions/pixel.
Symmetry refers to the ratio of the computational complexity of compression to that of decompression. Codec's are frequently designed with a greater computational load on the compressor than the decompressor, i.e., they are asymmetric. While this may be a reasonable strategy for "create-once, play-many" video sequences, it limits the range of applications for the codecs. Asymmetric compression techniques are not suitable for teleconferencing, for example, since teleconferencing requires essentially real-time processing and substantially equivalent compression and decompression rates.